Determining a roof's pitch is a fundamental roofing skill but, luckily for professionals and amateurs alike, it is one of the easiest skills to learn. Even for the complete roofing novice, the biggest obstacle to accurately determining your roof's pitch isn't the roof itself, but your fear of heights.
Mark a carpenter's level at 30 cm (12 inch) with a grease pencil.
Hold it perfectly level, with one end touching the roof. With a tape measure, measure from the roof surface to your 30 cm (12 inch) mark to determine your "rise." Alternatively, you can measure from the underside of a rafter in the attic, or the bottom of a barge rafter where it overhangs your house.
Measure, using a tape measure, from the ceiling or roof surface to your 30 cm (12 inch) mark to determine your "rise": how many centimetres your roof rises (for example, 14) for every 30 that it runs.
Express your roof pitch by placing the rise over the run: for example, 14/30. Also acceptable are 14-30, 14 on 30, 14 and 30, or 14 to 30.
You can also calculate your pitch with a pitch-and-angle calculator.
Irregular roofing, like shingles, may interfere with your results.